In recent years, the popularity of microwave ovens in the household has increased dramatically. This popularity is due, in part, to the ease and speed of microwave cooking of many foods, compared to conventional cooking. As the number of households having microwave ovens increases, the demand for suitable packaged and prepared foods adapted for microwave use also increases. Microwave cooking of some types of foods has experienced some consumer resistance and dissatisfaction. The characteristics and nature of microwave cooking are substantially different from conventional convection heating, and therefore some foods do not cook well in the microwave oven. One of the more notable disadvantages of microwave cooking is the inability to produce the highly desirable brown color on the surface of the food. The brown color is particularly desirable on meats, breads and pastries. Microwave cooking does not raise the surface temperature of the food to a high enough temperature, for a period of time long enough, to brown the food.
Efforts have been made to compensate for this lack of natural browning in the microwave, by either partially cooking the food in a conventional oven or by applying various sauces and other coatings which absorb or concentrate the microwaves. These coatings have met with only limited success in producing the desired browning of meats. These methods have also not proved to be particularly successful in microwave cooking of breads or pastries. These products require a brown surface to make the product acceptable to the consumer. Although they may be effectively cooked in the microwave, their bland appearance makes them undesirable to the consumer. The browning of foods has been the subject of much research for many years and is still not completely understood. The browning reaction of foods is commonly referred to as the "Maillard Reaction." The Maillard reaction or browning reaction can be defined generally as the action of amino acids and proteins on sugars. The carbohydrate must be a reducing sugar, because a free carbonyl group is necessary for such a chemical condensation reaction. The reaction proceeds with the eventual formation of melanoidins, which are brown-colored nitrogenous polymers and copolymers. The rate and extent of the browning reaction is influenced by a number of factors, including the particular amino acid or protein, the carbohydrate, and the presence of lipid. Different foods react at different rates and do not brown to the same extent. Foods rich in reducing sugars are usually very reactive, while foods having low concentrations of reducing sugars do not brown as fast or as much. Other factors which affect the browning reaction include temperature, pH, moisture level, oxygen, metals, phosphates, and sulfur dioxide.
In the Maillard reaction, the basic amino group is consumed, and so the initial pH of the system has an important effect on the rate of the reaction. The reaction slows down as the pH decreases, and therefore the reaction tends to be self-inhibitory as it proceeds. To maintain proper pH, a buffer can be added to the system. The pH of the food is dependent in part on the concentration of the amino acid and the amount of moisture in the food. When a large amount of water is present, most of the browning occurs by caramelization of sugars. At lower water levels and at pH levels greater than about 6, the Maillard reaction is the predominant cause of browning.
The previous efforts to overcome the inability of foods to brown during microwave cooking or heating are not completely effective in achieving a pleasing brown color in the microwave oven. In addition, these compositions are not particularly stable for extended periods of time, and this instability typically results in premature browning during storage. There is, therefore, a need for a browning composition that is shelf-stable and can be activated by microwave energy to produce the distinctive brown color and flavor associated with conventional cooking. The present invention is directed to such a browning composition.
The browning composition, according to the present invention, includes the reactants, essential to produce the Maillard reaction during heating, in a form that is stable at room temperature for extended periods of time. The stability of the browning composition is achieved by preparing a composition containing a film-forming component, a reducing sugar, and a liposome-encapsulated base or basic amino acid, the latter at a pH higher than the pKa of the alpha amino group. Therefore, the amino acid is maintained in its most reactive state, but is physically separated from, and thereby inhibited from direct contact with, the reducing sugar prior to release from the liposome. Nucleophilic condensation of amino acid and sugar aldehyde is facilitated when the liposomes rupture upon exposure to microwave energy, thus releasing the amino acid and allowing it to come into direct contact with the reducing sugar located outside the liposomes.
Liposomes are essentially closed lipid bilayer membranes in the form of vesicles or sacs containing an internal aqueous core. The liposomes are formed from an aqueous component and a polar lipid. The phospholipids are the common lipids used in preparing typical liposomes. The polar lipid forms a membrane by orienting its polar hydrophilic end toward the aqueous phase and orienting its non-polar end toward the center of the liposome bilayer. The structure of the liposome provides a unique carrier for water-soluble components entrapped in the aqueous core, which cannot otherwise be readily segregated in an aqueous medium.
Liposomes can be prepared by a number of known methods. Depending on the method employed, the liposomes formed can be either unilamellar vesicles, having a single lipid bilayer membrane, or multilamellar vesicles, having a number of concentric lipid bilayers. The multilamellar liposomes are generally the preferred form in most commercial applications, because multilamellar liposomes are able to encapsulate larger amounts of materials and are able to encapsulate larger molecules, including macromolecules.
In one of the early methods of preparing liposomes, a phospholipid, such as phosphatidylcholine, is suspended in an organic solvent which is then evaporated to dryness, resulting in a waxy phospholipid film on the wall of the vessel. An aqueous solution of the material to be encapsulated is added to the vessel and agitated to produce a dispersion of unilamellar liposomes.
Efforts to increase the volume of entrapped materials in liposomes have resulted in the formation of inverse micelles or liposome precursors. The precursors or vesicles contain an aqueous phase surrounded by a monolayer of lipid molecules oriented so that the polar heads are directed toward the aqueous phase. The liposome precursors are formed by sonicating a mixture of an aqueous solution and an amphiphilic lipid dissolved in an organic solvent. The water and the organic solvent are then evaporated in the presence of excess lipid. The resulting liposomes are then redispersed in an aqueous medium.
The physical characteristics of liposomes can be improved by including various additives in the lipid bilayer. For example, the permeability of the lipid bilayer membrane can be reduced by including a minor amount of cholesterol with the amphiphilic lipid, in order to enhance the orientation of the lipid and produce a more orderly array. The ordered arrangement of the molecules stabilizes the bilayer and reduces permeation of the encapsulated material.
An important physical property of liposomes, which is utilized in the heat-triggered, controlled release of Maillard browning reagents in the present invention, is the phase transition temperature of the membrane. Each type of phospholipid has its own characteristic phase transition temperature. At temperatures below the phase transition temperature, the lipid membrane is in a highly ordered, crystalline array. At temperatures above the phase transition temperature, the lipid melts, causing the membrane to rupture and the encapsulated ingredients to be instantaneously released.
Liposomes are generally stable structures, relatively simple to produce, and can be dispersed in most aqueous solutions. Liposome dispersions offer a convenient vehicle for delivering a component that cannot otherwise be easily dispersed in an aqueous solution. In addition, the structure of liposomes provides a means of protecting and isolating a component from the external environment, until the liposomes rupture.
The most common use of liposomes has been in the pharmaceutical field to deliver drugs to a specific site. To a lesser extent, liposomes have also been used in the food industry to introduce a particular component to a food composition during processing. An example of such applications in the food industry is the use of liposomes to encapsulate an enzyme in the aqueous layer, and the introduction of the liposomes during the cheese-making process to accelerate the ripening of cheese. Encapsulating the enzyme inside the liposome stabilizes the enzyme, such that it remains active longer during the process.
There is a need for a browning composition that is shelf-stable for an extended period of time and that does not undergo premature browning during storage. There is further a need for a microwaveable browning composition that can be applied as a thin, colorless coating to the surface of foods and that will remain adhered to the surface of the food during cooking or heating. The present invention is directed to a microwaveable composition that overcomes the limitations of the previous efforts to produce a browning composition suitable for microwave use.